Quasi-phasematching (QPM) employs a material with spatially modulated nonlinear properties. Compared with an ideal phasematched case (e.g., involving birefringent phase matching—BPM), QPM typically exhibits a lower conversion efficiency if the nonlinear coefficient is the same. However, QPM allows frequency conversion in isotropic materials or ones that exhibit little birefringence, and in some cases allows access to larger components of the nonlinear susceptibility tensor, with coupling of waves of the same polarization. QPM also has an advantage of eliminating spatial walkoff due to birefringence, enabling a greater degree of collinearity among output beams, and allowing use of longer crystals to obtain higher optical output powers. In effect, the conversion efficiency for QPM can be higher than that due to BPM.
Various techniques have been developed to engineer quasi-phasematching (QPM) in non-linear materials. For instance, periodic poling is a technique for obtaining quasi-phase matching of nonlinear interactions. Periodic poling involves a process (e.g., ferroelectric domain engineering) that generates a periodic reversal of the domain orientation in a nonlinear crystal, so that the sign of the nonlinear coefficient also changes. Another related approach can achieve ferroelectric domain reversals via thermal poling. A technique for engineering quasi-phase matching in non-ferroelectric crystals is semiconductor orientation-patterning. This technique allows frequency conversion to wavelengths well into the mid-infrared and even far infrared regime. Before these and other QPM materials can be utilized, it is desirable to verify that a modulation of the crystalline orientation has been achieved.
Although X-ray diffraction may seem like an obvious tool for inspection, the electron beams that are used are typically orders of magnitude larger (˜mm) than many domain widths. Additionally, typical X-ray diffraction cannot distinguish between domains of inverted orientation in any case. Thus, the conventional scheme for characterizing orientation patterning of semiconducting crystals usually involves cleaving the device along the grating direction. An anisotropic etchant is then applied to the surface, and scanning electron micrography (SEM) or optical microscopy is employed to detect variations in sidewall profiles. This technique can be destructive, risking catastrophic damage to the samples, and requires knowledge of the appropriate etchant for attacking different orientations of the particular semiconductor at different rates.